Roll stability control (RSC) is normally implemented only by means of an electronically controlled braking system (EBS). EBS involves the electronic control of an air brake system using electronic signals to allow air into the brake chambers to operate the brake system. EBS provides the platform for a variety of vehicle control features. RSC is used to prevent rollover accidents. RSC prevents overturning of a vehicle about the longitudinal axis, the most common rollover situation. RSC can be a part of a tractor EBS as well as a part of a trailer EBS.
In a normal braking system or in an Antilock Braking System (ABS), the brake pressure is mechanically provided by the driver operated brake valve. With EBS, the brake pressure is electronically controlled. Therefore, the EBS must be able to apply and release brake pressure by means of electrically operated solenoids. EBS requires the use of ABS wheel speed sensors (WSS) at each wheel, and also pressure modulator relay valves (MRV). The solenoids and valves of an EBS system are used for every braking operation of the vehicle, including service braking, and therefore must be very robust—more so than ABS hardware, which is used only in an ABS event, which is relatively rare.
RSC requires additional components beyond the EBS components as described above. For example, an EBS based RSC in a trailer requires more sensors in addition to the ABS wheel speed sensors and pressure modulator relay valves. These additional sensors may include a lateral acceleration sensor, a suspension air bag sensor, a pressure sensor on the control pressure side and a pressure sensor on the delivery side of each control channel.
On the brake pressure control side, RSC requires a modulator valve that is electrically activated to apply, hold and release pressure to an individual brake chamber independent from the normal braking situation. This 3-stage pressure control typically can be achieved only with 3 solenoids for each channel of the pilot controlled relay valves. These solenoids include a back-up solenoid for switching from mechanical to electrical braking, a hold solenoid for pressure hold and build up, and a release solenoid to vent the brake pressure to the relay exhaust.
The number of pressure sensors for EBS-based RSC for use in a trailer thus reaches 5. In addition, a lateral acceleration sensor is needed for measuring the actual lateral force. FIG. 9 shows an example of one type of EBS system used for implementing RSC. A main component of EBS is the pressure modulator relay valve (MRV), designed in one or in a two-channel version (as shown in FIG. 9). The 2-channel control modulator consists of 6 solenoids, 5 pressure sensors, 2 relay valves and an ECU. The 2 back up solenoids control the control pressure lines. The 2 hold solenoids control pressure hold and build. The 2 release solenoids control pressure release. The ECU is mounted on top of the modulator and includes data processing and control. All solenoids are 2/2 design, meaning 2 pneumatic lines controlled with two armature positions.
By processing the actual data as measured from the various sensors and the ECU's internal built vehicle reference speed, the ECU automatically evaluates the potential danger of overturning. If a potential for overturning is indicated, the ECU provides a braking test pulse with a low-pressure value to the curve inside wheels to identify the actual roll over tendency. If the danger of overturning is real, the ECU activates hard braking to the curve outside wheels and reduces the risk for overturning by reducing the vehicle's speed.
While the hard braking procedure is typically a full braking apply to the curve outside wheel, the test pulse to the curve inside wheels is a more sensitive operation. The real potential for overturning is only recognizable if the curve inside wheel slows in a predetermined characteristic manner when a relatively low and accurate pressure test pulse is applied. Only by using pressure sensors connected to the output delivery lines can the EBS assure the necessary accuracy of the actual brake pressure during normal braking and during the test pulse in a roll stability event.
To achieve the desired brake pressure value accuracy, the EBS as installed in a trailer needs 5 pressure sensors and 6 solenoids to provide the multiple functions of normal braking, ABS braking, and roll stability control.